This invention relates to locating impulse noise in a communications system, particularly but not exclusively a submerged telecommunications system.
With the now widespread use of data circuits on submerged telecommunications systems, telephone administrations are specifying minimum performances for impulse noise generation. The simplified supervisory circuits of modern system repeaters have no facilities to detect noise uncorrelated with the signal and it is not desirable to increase their complexity and cost to enable them to do so. This means that, if an installed system is giving unsatisfactory service on this account, it could be an onerous and very expensive procedure to make many cuts into the system to find the faulty part. With this in mind, expensive testing procedures, during the production of a system, are required. These test the repeater apparatus at the time of manufacture but subsequent additions, such as cable and joints, cannot be tested in the factory and the effects of any subsequent degradation cannot be taken into account.
Sources of impulse noise. There are three main sources of impulse noise in a repeatered submarine telecommunication system:
(a) The high voltage cable and joints and, more importantly, at the repeater, the entry joints, glands and small-diameter entry cable. It can be shown that the spectral density of this type of noise is independant of frequency, for a given charge transferred at the impulse, and, since it occurs more importantly at a repeater input, it bears a constant ratio to thermal noise.
(b) The high voltage blocking capacitor at the repeater input which forms part of the power separation filter of the repeater, including its seal and any other insulation directly in parallel with it. For a given charge transferred, this is a less important contributor than (a) since the capacitor itself largely decouples the repeater signal circuits from the impulse. The spectral density of the noise is greatest at the bottom end of the traffic band and falls off rapidly with frequency, for example, at 12 db per octave. Thus, it is unlikely to make most of the traffic band unserviceable--both because of its limited spectrum and because the signal relative level is high.
(c) The capacitance to sea of the body of the repeater, including any high voltage capacitor used to augment it. In principle, this is a much more prolific source of noise, being subject to scattered insulation paths with large exposed surfaces--such as perspex bars and a polythene shell. But the noise is well decoupled by the components necessary to provide a high singing-loop loss across the repeater--especially if a capacitor is fitted. This noise is not amenable to calculation (although a repeater prototype could be calibrated by a standard discharge) since the loop loss is set by ill defined parasitic components, but it should not be a problem in a well designed repeater. This noise is likely to be greatest at the extreme ends of the spectrum, i.e., where the loss from the center capacitance to the signal path is least.